Embedded atom method study of Cu deposition on Ag(111)

A new model is applied to the study of Cu deposition on Ag(111), It links elements from the embedded atom model and Monte Carlo simulations. In agreement with the experiment, the present results favor overpotential upon underpotential deposition. A diffusion coefficient of 1.93 × 10⁻⁶ cm² s⁻¹ is estimated for the system. The predicted structure of the adsorbed monolayer is more compact than the electrochemical observation, a fact which may be due to some simplifications of the model like neglecting the solvent and anion effects or the nature of the potentials employed.     

Atomistic kinetic Monte Carlo study of atomic layer deposition derived from density functional theory

To describe the atomic layer deposition (ALD) reactions of HfO₂ from Hf(N(CH₃)₂)₄ and H₂O, a three‐dimensional on‐lattice kinetic Monte‐Carlo model is developed. In this model, all atomistic reaction pathways in density functional theory (DFT) are implemented as reaction events on the lattice. This contains all steps, from the early stage of adsorption of each ALD precursor, kinetics of the surface protons, interaction between the remaining precursors (steric effect), influence of remaining fragments on adsorption sites (blocking), densification of each ALD precursor, migration of each ALD precursors, and cooperation between the remaining precursors to adsorb H₂O (cooperative effect). The essential chemistry of the ALD reactions depends on the local environment at the surface. The coordination number and a neighbor list are used to implement the dependencies. The validity and necessity of the proposed reaction pathways are statistically established at the mesoscale. The formation of one monolayer of precursor fragments is shown at the end of the metal pulse. Adsorption and dissociation of the H₂O precursor onto that layer is described, leading to the delivery of oxygen and protons to the surface during the H₂O pulse. Through these processes, the remaining precursor fragments desorb from the surface, leaving the surface with bulk‐like and OH‐terminated HfO₂, ready for the next cycle. The migration of the low coordinated remaining precursor fragments is also proposed. This process introduces a slow reordering motion (crawling) at the mesoscale, leading to the smooth and conformal thin film that is characteristic of ALD. © 2013 Wiley Periodicals, Inc.     

On the structure of the disordered Bi2Te4O11 phase

The structure of the disordered metastable Bi₂Te₄O₁₁ phase has been investigated using both neutron powder diffraction and reverse Monte Carlo (RMC) modelling. The average structure, of fluorite-type (space group ), is characterized by very high Debye-Waller parameters, especially for oxygen. Whereas the cations form a fairly well-defined FCC lattice, the oxygen sublattice is very disordered. It is shown that the local order is similar to that present in the stable monoclinic Bi₂Te₄O₁₁ phase. Clear differences are observed for the intermediate range order. The present phase is analogous to the “anti-glass” phases reported by Trömel in other tellurium-based mixed oxides. However, whereas Trömel defines anti-glass as having long range order but no short range order, it is shown here that this phase is best described as an intermediate state between the amorphous and crystalline states, i.e. having short and medium range order similar to that of tellurite glasses and a premise of long range order with the cations only.     

能量选择中子成像技术及其应用

能量选择中子成像技术是利用特定范围波长(能量)的中子进行成像。在热/冷中子范围内(<25meV),能量选择中子成像技术主要基于布拉格边效应和衍射机制,相比常规中子成像技术不但可以显著提高图像对比度,而且能分析应变、应力、织构。在超热中子范围内(>1eV),能量选择中子成像技术主要基于中子共振吸收,中子截面随能量变化是同位素特有的,存在明显的共振吸收峰,因此可以进行同位素的“指纹”识别。能量选择中子成像技术在工程、材料、化学、物理、生物、考古等众多科研领域中有着非常广阔的应用前景。     

Monitoring of gamma emission and neutron transmission during boron neutron capture treatment delivery

The ability to map boron and hydrogen distributions in the body is paramount to the success of boron neutron capture therapy (BNCT). We investigated treatment-time quantitative mapping of these distributions by detecting (i) 0.48 MeV de-excitation photons from neutron capture by boron-10; (ii) 2.22 MeV photons from neutron capture by hydrogen; and (iii) transmitted neutrons. Monte Carlo simulations reported no detectable difference when ¹⁰B in tumour was varied from 0 to 50 ppm, and when the tumour size was varied from 0.0 to 9.5 cm³.     

Monte Carlo方法模拟胶体沉降过程中粒子半径的影响

运用Ｍｏｎｔｅ Ｃａｒｌｏ方法模拟了粒子在重力场中扩散控制的沉降过程，获得了不同粒径下的沉降图样，并用分维进行了分析。结果表明，随着粒子半径的增大，沉降图样堆积得越来越紧密，当粒径小于４３０ｎｍ时，分维数随粒径的增大迅速增大；大于４３０ｎｍ时，分维数增长缓慢，最后趋近于２。     

环氧树脂电子束固化过程中的电子能量传播机制

将环氧树脂辐射固化过程中的温度分布和辐射固化后的固化度分布与反应前的电子能量沉积模拟计算结果相结合, 探讨电子束在辐射固化过程中的能量传播机制. 结果表明, 辐射开始初期, 固化反应发生前, 电子能量在聚丙烯模具内环氧树脂体系中的沉积满足离子注入理论, 即电子能量沉积在距辐射表面一定距离处达到最大, 然后随辐射距离的增加沉积能量减小; 而在玻璃模具内的树脂体系中, 电子能量从辐射表面向里逐渐降低. 随体系中固化反应的发生, 最大电子浓度区域转移, 最终出现在临近最大电子沉积浓度区域辐射深度稍远的地方. 能量吸收和反应放热导致的升温不影响树脂固化度大小, 但会影响固化度分布.     

Free Energy of Deformation of the Radius of Gyration in Semiflexible Chains

The distribution function P(S) of the radius of gyration S, the corresponding elastic free energy A(S) and the mean force were computed from simulations based on the wormlike chain (WLC) model. The relation of the S‐conjugated elastic functions to the analogous functions based on the chain vector R and their connection to the statistical‐mechanics ensembles was elucidated. Simulation data revealed that available analytical functions for P(S) fail to predict the behavior of semiflexible chains. When the power‐law function P(S) was used instead, the exponents sizeably raised with stiffness at chain expansion. The exponents deduced from elastic compression of a chain agreed fairly with the scaling exponents for chain confinement into a sphere.

     

Driven Chain Macromolecule in a Heterogeneous Membrane-Like Medium

Monte Carlo simulations are performed to study the conformational relaxation of a large polymer chain driven into a heterogeneous (membranelike) substrate on a discrete lattice. Chains are created on trails of constrained self-avoiding walks (SAW) on the lattice. Kink–jump, crank–shaft, and reptation moves are used to move segments of chains. Short chains of length L _sc are driven by a field E ₁ toward an impenetrable substrate to design a membrane medium with mobile chain segments. A long chain of length L _lc is then driven by a field E ₂ into the membrane medium and is subsequently allowed to relax in a field E ₃. Radius of gyration R _g and end-to-end distance R _e of the long chain are examined. The relaxation of the conformation of the long chain and its magnitude is found to depend on the initial (predeposition) conformation of the chain, i.e., on E ₂. For a relatively relaxed initial conformation (at E ₂ = 0.1), the longitudinal component of the radius of gyration (R _gz ) is found to decay with the driving field E ₃ with a power law, R _gz E ₃ where 0.1 at low field (E ₃ 0.1) and 1/3 at high field E ₃ 0.1.     

Self-optimizing Diffusion Quantum Monte Carlo Calculation: the Potential Energy Curve of C_2

11NTRODUCTIONDiffusionquantumMonteCarlo(DMC)isoneofthesimplestofthevariousMonteCarlotechniquesavailabletosolvetheSchrodingerequation,forarecentre-viewofDMC(seeRef[1i).Foravarietyofsmallatomsandmolecules,DMChasbeenshowntobecapableofprovidinganestimateoftheground-state(nonrelativistic)energywithanaccuracycomparabletogoodqualityClcalculations,evenwhenarelativelysimpletrialwavefunctionisemployed.However,todate,thereareseveralobstaclesinaDMCcalculation:(1)BeforetheDMCcomputation,thepar…     

基于瞬发伽马谱学的元素重建算法

质子治疗中的瞬发伽马射线是质子和靶标之间核反应的产物,瞬发伽马射线的特征能量和强度可以用来确定靶中元素的种类和数量,在之前的实验中已经证明,无论反应截面多么复杂,一旦确定了被照射元素和入射质子的能量,元素浓度与伽马射线光子数之间就存在一定的线性关系。然而,这种线性关系很难应用于医学成像,而且氢的非线性行为迄今尚未研究。本文将这种线性关系推广到包括氢等非线性情况的混合元素材料,并提出了一种通用的数学形式,即基于瞬发伽马谱学的重建算法 (PGSRA)。PGSRA的基本假设是样品材料的PGS与元素的每摩尔伽马射线有某种关系。对于碳和氧,这种关系是线性的,而对于氢,这种关系是非线性的。由于2.23 MeV的伽马线来源于中子吸收辐射,我们仔细研究了氢非线性行为。利用蒙特卡罗模拟验证了碳、氧和氢的不同组合,如PMMA、戊二醇和乙醇二醇的线性和非线性关系。在这项工作中开发的PGSRA可能是PGS和医学成像之间的第一座桥梁。     

A sparse algorithm for the evaluation of the local energy in quantum Monte Carlo

A new algorithm is presented for the sparse representation and evaluation of Slater determinants in the quantum Monte Carlo (QMC) method. The approach, combined with the use of localized orbitals in a Slater-type orbital basis set, significantly extends the size molecule that can be treated with the QMC method. Application of the algorithm to systems containing up to 390 electrons confirms that the cost of evaluating the Slater determinant scales linearly with system size.     

Characterization and electrochemical deposition of natural melanin thin films

Melanin is an important class of biological pigments because of its distinct chemical and physical properties. The electrochemical deposition of natural melanin thin films was studied using two different techniques; constant potential and cyclic voltammetry along with a deposition time of five hours. The thin films deposited electrochemically on a fluorine-doped tin oxide conductive glass substrate using the constant potential method, exhibited faster growth rate and better adhesion to the fluorine-doped tin oxide working electrodes than those deposited using the cyclic voltammetry method. The thin films deposited on the fluorine-doped tin oxide conductor glass using the constant potential method were also more homogeneous than those deposited via the cyclic voltammetry technique. The increase of film thickness is related to the increase of electrochemical deposition time. Interestingly, the electrochemical deposition using the constant potential method had the advantage of consuming less electric charge. The physical and chemical structures of the melanin thin films were characterized using ultraviolet–visible absorption spectroscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction analysis. The ultraviolet–visible absorption spectra showed the correlation between the variation of deposition rates of melanin and the type of electrochemical technique employed as well as the thickness of the film. The average thickness of the film is 500 nm which absorb 40% of light in both type of films. The atomic force microscopy images illustrated the homogeneous deposition of the melanin molecules on the fluorine-doped tin oxide conductive glass substrate, indicating that the thickness of the thin films can be controlled. We estimated an average grain size of 14.093 Å. The ease of preparing such thin films of organic materials can open new avenues towards the use of soft conductors, in contrast to the complex preparation of industrial semiconductors.     

Free Energy of Polymers Confined in Open and Closed Cavities

Distinct differences between the thermodynamics of open and closed cavities are observed in confinement free energy of macromolecules as a function of chain length and cavity radius and can be of special importance in the case of processes in spatially heterogeneous confinements encountered in various nano‐ and biostructures. In treatments of the confinement free energy, special attention is given to the equilibrium conditions (a full equilibrium for free exchange of macromolecules between cavity and bulk solution or a restricted equilibrium with number of chains in cavity constant) and associated polymer concentration changes. Increased chain stiffness brings about additional effects and complexity, for which the first results are presented here.

     

Monte Carlo simulations for predicting the precision of results and for optimizing data acquisition schedules

Monte Carlo simulations can be used to determine the precision of an analytical method if the standard deviations of the component unit operations are estimated accurately. Alternative methods for estimating the standard deviation have been compared by evaluating the success of Monte Carlo simulations to predict the precision of a second-order rate constant determined by spectrophotometry and of an equivalent weight and acid dissociation constant determined by potentiometry. Monte Carlo simulation has also been used with simplex optimization to predict a data acquisition schedule which gives high precision in the equivalent weight determination. By comparison with a naive design, a 22-fold improvement was predicted. A 15-fold improvement was observed experimentally.     

Production and Neutron Irradiation Tests on a New Epoxy/Molybdenum Composite

Molybdenum powder was added in an epoxy matrix to increase the neutron-shielding capacity of pure epoxy. FLUKA and GEANT4 Monte Carlo programs were used to design new neutron-shielding material. After finding the epoxy/Mo ratio for the best shielding capacity, neutron equivalent dose rate measurements were performed. Results were compared to commonly used neutron-shielding materials such as paraffin, steel, concrete, and B₄C. The produced new sample has a high neutron shielding capacity and it can be used for neutron protection purposes.     

Risk assessment of inhalation exposure for airborne toxic metals using instrumental neutron activation analysis

In order to study the effects of air pollution, about 1,300 samples of airborne particulate matter (APM) were collected at suburban and industrial sites, in Daejeon, Korea from 1998 to 2006. The concentrations of carcinogenic (As and Cr) and non-carcinogenic metals (Al, Mn, and Zn) were determined by using instrumental neutron activation analysis (INAA). These long-term metal concentration data were applied to a risk assessment of inhalation exposure using Monte Carlo analysis (MCA).     

玉门稠油组分特征及其对结蜡行为的影响

为了研究稠油不同组分的特征及其相互作用,利用柱层色谱分离法、傅里叶红外光谱、差示扫描量热法（DSC）和偏光显微分析等表征方法及手段,对采自玉门油田的稠油样品进行了组分分离、分析,并对饱和烃组分结蜡行为的影响进行了研究。结果表明,稠油各组分相互作用可以有效抑制蜡晶的析出。饱和烃组分（A₁）中分别加入其他不同极性组分后,其结蜡行为与原油原始状态差异较大;A₁的析蜡点、析蜡峰温和析蜡量均有所降低。偏光显微分析发现胶质沥青质组分使A₁冷却结晶时的蜡晶颗粒数增多,尺寸相对减小,可以减弱蜡晶之间的联结强度,削弱蜡晶缔合而形成大块蜡晶聚集体的倾向。     

Monte Carlo study of three-dimensional organization of water molecules around DNA fragments

Interactions with water molecules are important for the stabilization of three-dimensional structures of nucleic acids and for their functioning. The first hydration shells of macromolecules can be considered as structural parts of nucleic acid. We performed a Monte Carlo study of systems containing a nucleic acid base or base pair with water molecules using improved potential functions. These potential functions enable experimental data on both single base–single water interaction energies and enthalpies of base hydration to be reproduced. Hydration shell structures of base pairs are dependent on the pair geometry. Structural elements of hydration shells can contribute to the pair stability and hence to the probability of mispair formation during nucleic acid biosynthesis. The distribution of water molecules around bases and base pairs is essentially nonhomogeneous.From the Proceedings of the 28th Congreso de Químicos Teóricos de Expresión Latina (QUITEL 2002).     

Surface diffusion of metal atoms on polymer substrates during physical vapour deposition

 The surface diffusion of physical-vapour-deposited metal atoms on thermoplastic polymer substrates was investigated. In accordance with the hypothesis of the “classical” atomistic diffusion model, diffusion coefficients are derived from a Monte Carlo simulation. Because the “classical” atomistic diffusion model neglects the desorption of the metal atoms, the absolute diffusion data obtained in our investigations should only be considered as rough estimates. It is more the intention of our work to present relative values in order to correlate the metal surface diffusion on polymer substrates with their physical states (morphologies and surface dynamics). As expected, the diffusivity of metal atoms is strongly influenced by the chemical affinity (“reactivity”) between the metal atoms and the polymer substrate. Furthermore, the diffusivity strongly depends on the physical state of the polymer substrate. On polymer surfaces above the glass-transition temperature the surface diffusivity of metal atoms is 1 order of magnitude higher than the diffusivity below the glass-transition temperature. Received: 9 April 1999/Accepted in revised form: 21 October 1999